Science Behind The Magic | Frame Geometries

Bicycle geometry affects more than just fit, it drastically changes how the bike rides. Different manufacturers sell the same fit size frame with significantly different ride characteristics based on head tube angles, chain stay lengths, bottom bracket heights, and numerous other dimensions.

Lets break down a few of the most important dimensions, how they affect ride quality, and compare some different industry leading frame designs to give you a better idea of how they vary. This article will not show you how to size a bicycle for personal use, but it will show how the same size bike can have very different designs and dimensions depending on its intended use.

Headtube angle is the angle formed between the horizontal plane and the fork steerer/headtube. Commonly you will hear people talk about slacker angles, which are seen on all-mountain and downhill bikes. Smaller headtube angles push the front wheel further out in front of the bike, providing less snappy steering, but a more stable feel at high speeds while descending. Cross-country bikes have larger angles, providing tighter steering response, good for slower speeds and while climbing.

When discussing headtube angles, it is always prudent to mention fork offset. The fork offset can be best described as the distance from the axis through the center of the steerer tube to the center of the axle. It is a positive value that, when increased, will lengthen the wheelbase of the bike, pushing the front wheel further out, resulting in decreased trail and snappier steering.

The combination of headtube angle and fork offset can be measured by trail. The steering torque that must be reacted to by the rider is equivalent to the trail multiplied by the lateral forces being exerted on the wheel. So by changing headtube angles, offset, and even the crown to axle length of the fork, trail is changed, ultimately changing the amount of torque required at the handlebars to turn the wheel and tire. The following diagram will show the angles discussed

To summarize, changing these steering dimensions to increase trail, will increase the torque input required by the rider to change directions, resulting in a more self-centering, stable feeling bike. Or if a nimble climbing bike is desired, altering geometries to reduce trail will decrease required steering torque, making the steering quicker and snappy but requiring more frequent rider input to keep the bike going in a straight line.

Bottom bracket height, the distance from the ground to the center of the bottom bracket, will greatly affect a bike’s corning ability. Higher bottom brackets generally suggest a higher center of gravity. Ideally, we want the center of gravity as low to the ground as possible, to keep the bike planted as we lean it over through a corner. This is the same as comparing a lowered sports car going through a corner to a truck or SUV. The higher center of gravity becomes further off-axis at higher speeds in a truck than the sports car, requiring more driver input to correct it, and slower speeds to prevent rollover. But when designing a frame, there must also be enough clearance that the pedals and frame aren’t constantly striking obstacles in the trail. Another element is introduced when compressing suspension, especially in full suspension bikes, as the suspension is compressed, the bottom bracket will become significantly closer to the ground. This is best illustrated in Vital’s G-Out Project, showing pictures of bicycle suspension as they reach the limits of their travel.

Wheelbase is the distance from axle to axle. Longer wheelbases provide riders with a more stable feeling and help keep the bicycle traveling in a straight line. Longer wheelbases are preferred in downhill riding, but aren’t ideal in tight, technical turns when it is necessary to “throw” the bicycle around a corner. The shorter wheelbases will make the bicycle feel more nimble and agile in these situations. Chainstay lengths also affect wheelbase and can change ride characteristics similarly. There is a delicate balance between altering overall wheelbase and just chainstay lengths though, as the rear triangle design also determines suspension characteristics and behavior.

The table below compares dimensions for Specialized cross-country, all mountain, and downhill models to show how the numbers differ across the different riding styles and purposes.

Stumpjumper Hardtail 29er

Enduro 650b

Demo 8 650b

Seat Tube Length

470mm

463mm

419mm

Top Tube Length

620mm

610mm

613mm

Chainstay Length

430mm

422mm

430mm

Bottom Bracket Height

310mm

352mm

343mm

Seat Tube Angle

73.5 degrees

74.5 degrees

76 degrees

Head Tube Angle

71 degrees

65.5 degrees

63.5 degrees

Wheel Base

1105mm

1188mm

1233mm

This table compares three brands of all mountain bikes with similar travel and intended purpose to show how Specialized, Intense and Pivot designs differ.

Pivot Mach 6

Specialized Enduro 650b

Intense Tracer T275

Seat Tube Length

483mm

463mm

485mm

Top Tube Length

607mm

610mm

616mm

Chainstay Length

431mm

422mm

432mm

Bottom Bracket Height

345mm

352mm

343mm

Seat Tube Angle

72.3 degrees

74.5 degrees

74.5degrees

Head Tube Angle

66.3 degrees

65.5 degrees

66.5 degrees

Wheel Base

1154mm

1188mm

1175mm

When looking at the above tables, and other bicycles, the wheelbase, headtube angle, and BB height are good numbers to focus on. These differences will result in the most significant changes in feel while riding. Here’s an idea; measure or find your bike geometry and a friends. If you can find someone with a significantly different frame within the same category of riding, try swapping and pay attention to the differences while you ride the same sections of trail. The more bikes you ride of varying designs, the easier it will be to narrow down your preferred feel. Pretty soon, after riding enough bicycles, you will know exactly what a bike will feel like, just by looking at the published geometry.

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About the Author

Brett Murphy grew up in Idaho where he discovered the addictive nature of riding mountain bikes, only to travel out to California to study mechanical engineering and find out that riding skinny tires was just as habit-forming. Now he splits the time between the two with an equal-opportunity mileage policy.